Recently, radio communication devices using wideband millimeter waves of 30 GHz to 300 GHz have become widespread. A radio communication technique using a millimeter wave is specifically applicable to radio transmission of high definition images or gigabit level high speed radio data communication. For example, the technique is disclosed in K. Maruhashi et al., “60-GHz-band LTCC Module Technology for Wireless Gigabit Transceiver Applications”, IEEE International Workshop on Radio-Frequency Integration Technology, Digest, pp. 131-134, December 2005, and K. Ohata et al., “1.25 Gbps Wireless Gigabit Ethernet Link at 60 GHz-band”, IEEE MTT-S International Microwave Symposium, Digest, pp. 373-376, June 2003.
However, a high frequency millimeter wave has a good property for advancing in a straight line and this may cause problems when radio transmission is tried within an indoor area. In addition, the millimeter wave has drawbacks in that a significant signal attenuation is caused due to the human body or the like. This may cause a difficulty in transmission, say, shadowing, and thus result in over-the-horizontal communication in cases where a human body exists between a transmitter and a receiver within an indoor area. The shadowing problem results from changes in propagation environments caused due to the high frequency of the radio wave and radio wave having a good property for advancing in a straight line, and is not limited to a millimeter wave band (having a frequency of 30 GHz or higher). Although it is not clear, the frequency at which the above-mentioned changes in propagation environments occurs is known as the level of approximately 10 GHz. Meanwhile, according to “Propagation data and prediction methods for the planning of indoor radio communication systems and radio local area networks in the frequency range 900 MHz to 100 GHz,” ITU-R, P.1238-3, April 2003, power loss coefficient indicates the volume of attenuation of wave with respect to the distance during propagation ranges 28 to 32 for the frequency range 0.9 GHz to 5.2 GHz, while the coefficient is 22 for the frequency of 60 GHz within an office. Since the coefficient is 20 in case of free space loss, it seems that propagation is affected less by scattering or diffraction at a high frequency, for example, 60 GHz.
To overcome the above-described drawbacks, for example, Japanese Patent Application Laid-Open No. 2006-245986 discloses, as shown in FIG. 1, a system in which a plurality of receiving parts 123 and 124 are arranged in receiving device 122 so as to form a plurality of transmission lines between transmitting device 121 and receiving parts 123 and 124, and transmission can be performed through one of the transmission lines even when the other transmission line is blocked.
As another solution to the above-mentioned drawbacks, Japanese Patent Application Laid-Open No. 2000-165959 discloses a system in which a reflector is mounted on a wall or a ceiling and a number of transmission lines are provided.
The method disclosed in Japanese Patent Application Laid-Open No. 2006-245986 is not available in cases where the vicinity of the transmitting device is blocked or where the receiving parts are all blocked. Moreover, the method disclosed in Japanese Patent Application Laid-Open No. 2000-165959 necessitates particular user consideration, for instance, the reflector should be disposed, taking into consideration the arrangement of the transmitter and the receiver.
However, a recent study on propagation characteristics of millimeter waves has found that there is a possibility of using a reflected wave even when no reflector is installed by intent.
FIG. 2 illustrates a system configuration using a wide angle antenna, and FIG. 3 illustrates an indoor delay profile of the system using the wide angle antenna shown in FIG. 2.
In the system using the wide angle antenna shown in FIG. 2, the principal wave which has arrived first has the largest received power, as shown in FIG. 3. The delayed waves including second and third waves arrived thereafter have small received power. These second and third waves are those reflected from a ceiling or a wall. This situation is significantly different from the propagation environments of radio waves of 2.4 GHz have a poor property for advancing in a straight line and that are used in a local area network, for example. At the frequency of 2.4 GHz, it is not easy to clearly isolate the arrival direction of radio wave due to effects of diffraction and multiple reflection. In the meantime, for a millimeter wave having a good property for advancing in a straight line, the arrival direction of wave is relatively clear, however, the number of delayed waves is restricted and a receiving level thereof is low.
Therefore, in case where a direct wave is blocked, it is required that a receiving level is ensured in the direction of reflecting a narrow beam having a high directive gain in order to continuously perform transmission using a reflected wave, as shown in FIG. 4. To eliminate user concern in regard to blockage of radio waves or concern about being positioned between a transmitter and a receiver, there is a need for a beam forming technique for dynamically controlling a narrow beam.
For beam forming, an array antenna needs to be constructed. An array antenna can be provided in a small area with the millimeter wave of short wavelength (for example, 5 mm at the frequency 60 GHz), and a phase shifter array or an oscillator array for such an array antenna is developed. For example, the technique is disclosed in J. F. Buckwalter et al., “An Injected Subharmonic Coupled-Oscillator Scheme for a 60-GHz Phased-Array Transmitter”, IEEE Transactions on Microwave Theory and Techniques, Vol. 12, pp. 4271-4280, December 2006, and S. Alausi et al., “60 GHz Phased Array in CMOS”, IEEE 2006 Custom Integrated Circuits Conference, Digest, pp. 393-396, San Jose, September 2006.
In an indoor millimeter wave system, problems arise when a direct wave is blocked and when radio transmission is continuously performed by a reflected wave.
It is desirable to reduce a data transmission interrupt time when the wave that is in use, whether it is a direct wave or a reflected wave, is converted. This is particularly desirable in the transmission of non-compressed image data, requested to have real-time property. In case of using a reflected wave, it is required to increase the directive gain of an antenna by narrowing antenna beam width to improve receiving intensity.
However, as antenna beam width becomes narrower, the beam searching direction (step) increases, and searching for and setting the beam direction takes time, thereby increasing the data transmission interrupt time. Accordingly, there exists a strong need for a method of setting a beam direction which can reduce data transmission interrupt time. In the meantime, a device capable of buffering data necessitates an extremely large memory when a data transmission interrupt time is increased, resulting in impracticality.